This invention relates to PVC stabilizers. In particular, it relates to a method for making an alkyltin thiocarboxylic acid ester from the corresponding alkyltin halide and thiocarboxylic ester. More particularly, it relates to a method for continuously synthesizing such an ester in an aqueous medium in a single vessel with extremely vigorous agitation.
Because of the drawbacks associated with the use of an organic solvent in batchwise preparations of such esters in the past (e.g., diminished yields based on the space and time allotted to the process), some have turned to methods in which the product is still made batchwise but with the elimination of the organic solvent. These, too, have their shortcomings. If the reaction mixture is strongly alkaline at the beginning of a batch, the pH decreases only gradually as the alkyltin halide is introduced; such conditions favor the hydrolysis of the starting materials. To minimize such hydrolysis, low temperatures are necessary during the rather long holding times associated with batchwise procedures. Side reactions such as the formation of alkyltin oxide by the reaction of ammonium hydroxide (an acid acceptor) and the alkyltin halide, and the base-catalyzed hydrolysis of the thiocarboxylic ester cause loss of desired product and produce materials which often cause the formation of a tenacious, stringy emulsion, the breaking of which requires a rather difficult and wasteful filtration.
Maul et al (U.S. Pat. No. 4,554,368) proposes to provide a continuous process for the preparation of these esters which comprises carrying out the reaction continuously in a system consisting of 1 to 5 agitated vessels with an average dwell time of 1 to 60 minutes at a constant pH value in the range from 3 to 8 and at a temperature from 40.degree. to 80.degree. C. Maul teaches that it is convenient to initiate the reaction batchwise, pumping the alkyltin chloride and the thiocarboxylic acid ester in the required molar ratio into the first reactor to fill it halfway before an aqueous solution of the base is added with stirring to raise the pH to the desired value. Then, the continuous addition of base, ester, and chloride is started and the volume of the reaction mixture is controlled by pumping it into a second agitator vessel. It is not clear that the continuous process described by Maul in terms of two agitator vessels would be successful when only one agitator vessel is used.
There remains a need, therefore, for a continuous process for the preparation of alkyltin thioglycolic acid esters from the corresponding halides and esters in an aqueous medium which produces a two phase product mixture which is substantially instantaneously separable in the absence of demulsification means.
There remains a further need for a process for the preparation of alkyltin thioglycolic acid esters from the corresponding halides and esters which is continuous from the start for an indefinitely long period wherein the use of but one agitator vessel for the reaction dramatically increases the yield of product on the basis of the reactor volume employed and the time expended for the process.